Bacteria: Bacilli, Spirilla and Cocci

The spiral bacteria are spirilla (singular form is spirillus) and the rod-shaped bacteria are bacilli (singular form is bacillus). The coccus (plural form is cocci) bacteria are spherical and are usually found in clusters or chains. Bacilli have a large surface area that helps them take in nutrients, but they may dry out easily. Cocci do not dry out as quickly and retain water, but they absorb nutrients slowly. Spirilla have flagella at both ends, allowing them to move like a corkscrew, and they are capable of moving faster than other bacteria. Their rigid cell wall gives them their shape.

An example of cocci is Staphylococcus hominis, and it occurs commonly on human and animal skin harmlessly, but it may occasionally cause infection in patients with weak immune systems. An example of bacilli is Escherichia coli, and these live in the intestines of animals and decompose undigested food. They are usually helpful or harmless, but some may cause food poisoning in their hosts. An example of spirilla is Leptospira interrogans, and these bacteria can cause leptospirosis, a disease which may lead to liver damage, but humans are accidental hosts and animals are most commonly infected.